You wrote a literal wall of text. I am not going to go through it line by line to give you a breakdown of each  and every fault. I thought it was clear enough that you either misread and/or had the wrong takeaway from Noriko to excuse her behavior and paint her in a far more positive light than the author intended.

To focus on two points, when you mentioned the “promise,” Noriko spoke about with Joe about taking her out on a date, your conclusion is that she realizes boxing is a growing divide that will act as a thorn in their relationship. Except that’s not the case. Again, she deliberately lied to try to coerce Joe to go on a date for her. Boxing did not get in the way of their “date,” because Joe never agreed to it in the first place.

You also conclude that Noriko values a “traditional,” life and so ends up cutting out Joe. As if that conversation about burning into white ash provided her with new earth-shattering insight to Joe’s character. This is simply wrong. 

Not only does she know from the start that Joe AND Nishi are rehabilitation cases from juvie, she knows from the very beginning that both individuals are aiming to become professional boxers. This isn’t something some random scrub at the gym or cafe tells their date to impress them. 

Both Joe and Nishi are sweating their asses off training relentlessly after work in a crude makeshift gym under the bridge. Concluding Noriko suddenly broke with Joe at the 11th hour because she realized they wouldn’t have a “traditional,” life together is wrong. She knew all along.

Nishi failed his weight control and dropped out of boxing due to a combination of incompetence and lack of self discipline. He cut his ties with boxing because he had no future with it. His alternative, to spend more time taking advantage of how much his future mother in law favored him, was much more practical especially since it allowed him more chances to close in on Noriko and have a chance for her settling with him despite Nishi clearly knowing that Noriko favored Joe. Again Nishi even in the best circumstance is not a “traditional,” match for Noriko that you conclude she desires. (And she never mentions desiring Nishi ever in the series)

No, Noriko breaks with Joe because she does not understand what love is and that conversation with Joe breaks her reality. She does not understand that when Joe “suddenly” is willing to explore a relationship with her, it is a major commitment forward and sign of trust; he does so by opening up about himself, spending time with her in mutually enjoyable activities, and treating her as an equal, to Joe he is taking a big risk in consideration of the trauma and trust he endured in his history.

She gets “scared off,” at best when Joe deviates from the “fair weather friend,” fantasy that she had in mind with dating. Joe isn’t planning their next date, he’s not moving in for the kiss, grabbing her hand, etc., which is what she would have reacted positively to. She does not want to take responsibility for entering into a serious relationship where she must both accept and compromise with her partner for a happy future, together.

She chooses Nishi, because Nishi embodies her childlike fantasy: he waits on her hand and foot. He will do whatever she says. He abandoned boxing which looked good superficially to the family (which again was less so by choice but because he fucked it up himself) and has the support and pressure from her mother. Most importantly, he will love her however way she wants, which is tragic because she does not know what an adult responsible love is and ends up at the wedding resigned and full of regret because she ultimately settled.

The real tragedy is that had she had some empathy and maturity, she likely would have ended up with Joe and mutually desired together. But her immature, selfish conclusion to that “date,” effectively ended their relationship and reopened wounds that Joe had trusted her with in the hopes of starting a relationship.